In cellular phone services in recent years, in addition to voice calls, a demand for data communication is expanding with an increasing number of subscribers (mobile stations), and therefore it is important to efficiently use resources such as frequency channels and improve communication speed. For example, in a GSM (Global System for Mobile communications) system which is becoming popular mainly in Europe and Asian regions, services corresponding to high-speed communication called “GPRS” (General Packet Radio Service) have started. In GPRS, with respect to a link between mobile stations and a base transceiver station, instead of allocating dedicated frequency channels to a specific mobile station, the same frequency channel is shared by a plurality of mobile stations, and, when data to be transmitted to the other party exists in either the mobile station or base transceiver station, time slots are allocated to a specific mobile station every time. This scheme improves utilization efficiency of frequency channels.
Moreover, by using a multi-slot transmission scheme which allocates a plurality of time slots in a data frame (8 time slots) to the same mobile station, improvement of downlink speed from the base transceiver station to the mobile station is realized.
For example, FIG. 1 shows an allocation of time slots for a mobile station. Here, representative classes up to class 12 are excerpted and described from 29 multi-slot classes described in a GSM specification “digital cellular telecommunications system (Phase 2+); Multiplexing and Multiple Access on the Radio Path (3GPP TS 05.02 ver 8.11.0 Release 1999).”
FIG. 1 shows a maximum number of time slots that can be allocated to a downlink (reception), uplink (transmission) and a maximum number of time slots that can be allocated to the downlink and uplink together for a mobile station which corresponds to each multi-slot class. For example, it is possible to allocate a maximum of 5 slots to the downlink and uplink together, a maximum of 4 slots to the downlink and a maximum of 4 slots to the uplink within 1 frame for the mobile station of class 12.
Furthermore, the respective mobile stations share time slots allocated to multi-slot classes, demodulate all data in the time slots, and then decide whether the TFI (Temporary Flow Identifier) in header information of the data indicates the subject mobile station or not, and discard the data if the data is directed to other mobile stations. In this way, each mobile station receives time slots allocated to the multi-slot class. According to the GSM specification “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station(MS)—Base Station System (BSS) interface; Radio LinkControl/Medium Access Control (RLC/MAC) protocol (3GPP TS 04.60 ver 8.18.0 Release 1999),” a TFI is described in 5 bits, and therefore one time slot can be shared by a maximum of 32 mobile stations.
Next, when the same frequency channel is shared by a plurality of mobile stations in a cell covered by a specific base transceiver station, the distance between the base transceiver station and each mobile station differs from one mobile station to another, and therefore it is necessary for the base transceiver station to carry out transmission to each mobile station with transmit power so that the farthest mobile station in the cell can receive data in predetermined quality. In this case, unnecessary power is radiated to the cell and interference with cells covered by adjacent base transceiver stations becomes a problem.
Therefore, an interference measures in GPRS will be explained using FIG. 2. FIG. 2 is a GSM network configuration.
In FIG. 2, a GSM network is configured with telephone network 11 of fixed-line phones, mobile services switching center (MSC) 12, base station controllers (BSC) 13, 14, 15, base transceiver stations (BTS) 16, 17, 18 and mobile stations (MS) 21, 22, 23, 24, 25 which exist in respective cells 19, 20 covered by base transceiver stations 17, 18.
The GSM system is provided with at least one mobile services switching center 12 and mobile services switching center 12 is connected to telephone network 11. A plurality of base station controllers 13, 14, 15 are provided below mobile services switching center 12 and at least one base transceiver station 16, 17, 18 is provided below base station controllers 13, 14, 15 and communication is carried out between the base transceiver stations. Furthermore, radio communication is carried out between base transceiver stations 17, 18 and mobile stations 21, 22, 23, 24, 25 in cells 19, 20 covered by the base transceiver station 17, 18. In FIG. 2, communication is possible, for example, between mobile station 22 in cell 19 and mobile station 24 in cell 20 or between mobile station 23 and telephone network 11. When transmission is performed to all the mobile stations with the same output power when the distances between mobile stations 21, 22, 23 in cell 19 and base transceiver station 17 differ from one another, transmit power is set based on the farthest mobile station, and therefore power which would be unnecessary under normal conditions is sent to the same frequency channels, which results in a problem of interference against adjacent cells. Thus, GPRS realizes downlink transmit power control (hereinafter, referred to as “power control”) to corresponding mobile stations according to the distance between base transceiver station 17 and mobile stations 21, 22, 23. Specific examples of power control include a method of reporting a decrement value (0 to 30 dB) of transmit power from a broadcast channel (BCCH: Broadcast Control CHannel) to a mobile station using a P0 parameter in a resource allocation message of the downlink which is transmitted on a control channel. BCCH is an important channel which all mobile stations existing in the cell should refer to and is transmitted with a sufficient transmission level (POWbcch: a fixed value) that even the mobile station at the maximum distance from the base transceiver station can reproduce data reliably.
In a system which supports multi-slot transmission and carries out power control, if data transmission of higher priority to another mobile station interrupts, or a control message such as recognition message for uplink data transmission for another mobile station interrupts multi-slot transmission to a specific mobile station, the received signal strength indicator (RSSI) with which the specific mobile station receives data fluctuates significantly. For this reason, to prevent the mobile station from saturating or to maintain the reception quality to a predetermined value, it is necessary to perform gain switching between adjacent time slots at high speed.
However, with a direct conversion reception apparatus which is a mainstream as a reception section configuration of the mobile station at present, DC offset voltage (hereinafter, referred to as “offset voltage”) is produced due to gain switching in accordance with RSSI fluctuation and there is a possibility that a reception apparatus may be saturated, and therefore it is necessary to perform gain switching immediately before receiving demodulated data and then calibrate the offset voltage at high speed (for example, see Patent Document 1.)
FIG. 3 is a block diagram of conventional offset voltage calibration circuit 30. In FIG. 3, offset voltage calibration circuit 30 is configured with low noise amplifier 31, quadrature demodulator 32 that converts the frequency of a radio frequency to a baseband, 90-degree phase shifter 33 that outputs 2 signals having a phase difference of 90 degrees for quadrature demodulator 32, analog baseband circuit 34 made up of variable gain amplifiers and low pass filters, voltage calibration circuit 35 that calibrates offset voltage of analog baseband circuit 34 and digital signal processing section 36 that converts the signal received from analog baseband circuit 34 to a voice signal or data signal and transmits a calibration start signal to voltage calibration circuit 35 via a decoder. Voltage calibration circuit 35 executes calibration operation for a certain period using a calibration start signal as a trigger immediately before the frame and pauses during the frame. Furthermore, in the calibrating period, voltage calibration circuit 35 separates a capacitor from the signal line to improve the calibration response speed.
Furthermore, as a gain setting method for the reception apparatus in multi-slot transmission, there is a method whereby demodulation processing is performed in multi-slot transmission at a fixed gain based on RSSI averaged over a plurality of time slots (for example, see Patent Document 2.)
FIG. 4 is a block diagram of reception apparatus 40 that supports conventional multi-slot transmission. In FIG. 4, reception apparatus 40 is configured with RF input section 41, automatic gain control circuit 42 that controls the gain of RF input section 41, sampling circuit 43, digital signal processor (hereinafter, referred to as “DSP”) 44 and control section 45.
Sampling circuit 43 periodically samples RSSIs of a plurality of time slots received by RF input section 41 and transmits the sampled RSSIs to DSP 44. DSP 44 generates an average RSSI of each time slot included in 1 frame and transmits the result to control section 45. Control section 45 further averages the average RSSIs of all received time slots and obtains a gain value. Then, control section 45 transmits the obtained gain value to the automatic gain control circuit as an AGC signal and performs reception operation.
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-211098
Patent Document 2: Japanese Patent Application Laid-Open No. 2003-46424